The present invention relates to an image projection system comprising an illumination system, a modulation system having at least two reflecting image display panels of the non-diffusing type for modulating light generated by the illumination system in conformity with image information to be projected, and a projection lens system. The image projection system includes an element having a color-separating as well as a color-recombining effect, and a polarizing beam splitter which is situated between the illumination system and the element and between the element and the projection lens system.
A reflective image display panel of the non-diffusing type is understood to mean a reflective liquid crystalline image display panel of the non-diffusing type or a DMD or the like.
An image projection system comprising two or three reflective image display panels may be given a very compact construction if the color separation and the color recombination are effected by one and the same optical system. The optical system may comprise, for example, a polarizing beam splitter. Since the other optical elements of such an optical system are situated between the reflective display panels and the polarizing beam splitter, it is undesirable that a change of polarization would be effected by the color-separating and color-recombining element. However, this is the case in practice, so that the ultimate image has a too low contrast and strong color deviations for all luminance levels between white and black. This is caused by the polarization-dependent transmission of the color-separating faces of the color-separating element and the geometrical decomposition of the polarization vector on all oblique faces due to non-perpendicular incidence. Each color channel causes a specific change of polarization as a function of the wavelength and the direction of propagation through the color-separating element. In addition to polarization changes of the light, light having a given direction of polarization may be reflected in an unwanted direction, In addition to strong color shifts in a color channel, this also causes unwanted optical crosstalk when light having a certain wavelength reaches a reflective display panel which is meant to modulate light of another wavelength.
The cause of the above-mentioned polarization effects is found in the relatively large angle of incidence on the color-separating faces of the color-separating element. Dependent on the element used, this angle of incidence may be between 10-20.degree. and 45.degree.. For a large angle of incidence, it is difficult to have an equal transmission characteristic of a color-separating coating for both s-polarized and p-polarized light. The smaller the angle of incidence, the smaller the above-mentioned problem will be. For small angles of incidence, the difference of transmission between p and s-polarized light can be minimized more easily. But also in this case, for example for a plumbicon prism, there is still a change of polarization.